1. Technical Field
The present invention relates to thermal processing in general, and, in particular, to a method for forming printed conductors on a flexible substrate.
2. Description of Related Art
Printed electronics is the convergence of the semiconductor industry and the printing industry. The notion of printing electronic circuits instead of printing reading materials is seductive to printers as they can see the potential for performing higher value print jobs without making major changes to their equipment. By the same token, electronic circuit manufacturers also view the notion of printing electronic circuits as equally seductive because it allows them to fabricate electronic circuits in large volumes at a relatively low cost.
One of the goals of printed electronics is the fabrication of electronic components, such as transistors, capacitors, resistors, etc., on low-temperature substrates, such as paper and plastic, via printing technology. One technique of fabricating electronic components is to print a dispersion of a particulate precursor directly onto a substrate. After printing, the material is then thermally cured to sinter the precursor particles into the final structure.
For example, silver nanoparticles can be printed onto a polyethylene terephthalate (PET) film or paper sheet via a screen, flexographic, gravure or inkjet printer. Afterwards, the nanoparticles are sintered using an oven operating at a relatively low temperature to form a highly conductive metal trace that is mesoporous or foamed. Printed silver is very reliable primarily due to the fact that it does not appreciably oxidize. Printed silver not only retains its bulk electrical conductivity, but its contact resistance remains low over time. Mechanically, printed silver is very stable, which makes it compatible with flexible substrates.
In order to reduce costs, copper is preferred over silver. Unfortunately, the comparatively inexpensive printed copper will oxidize over time. The copper oxidization causes multiple problems. When copper oxidizes, it expands and the expansion can cause delamination from the substrate and stress-corrosion-cracking in the inorganic film. In addition, since copper oxide is not conductive, both the bulk resistance and the contact resistance are increased. Finally, copper oxide is brittle, which makes it incompatible with a flexible substrate. This problem is exasperated when the flexible substrate is repeatedly flexed.
Consequently, it would be desirable to provide a method for forming an inexpensive printed conductor, such as copper, on a flexible substrate.